Dynamic setting of threshold voltage

Create a table using the threshold data measured above. During the in-clip measurement, the corresponding data are taken from the table according to the number of the two switches closed, and their sum is sent to the D/A converter to form the threshold voltage. For pen clip measurement and pen measurement, only the threshold data of one switch needs to be loaded because the measurement path only passes through the analog switch of route .

In addition, because the circuit itself (D/A converter, voltage comparator, etc.) has errors, and the actual measurement between the test fixture and the measured pin has contact resistance, so the actual load threshold voltage should be determined in accordance with the above method based on the threshold value plus a correction, so as not to misjudge the path as open. However, the increased threshold voltage will drown the small resistance, that is, the small resistance between the two pins is judged as a path. Therefore, the correction amount of threshold voltage should be selected reasonably according to the actual situation. Through the experiment, the detection circuit can accurately judge the resistance value of more than 5 ohms between the two pins, its accuracy is obviously higher than multimeter.

Several special cases of measurement results

1. The influence of capacitance

When a capacitor is connected between the measured pins, it should be a circuit breaker relationship, but the measurement path charges the capacitor at the moment the switch is closed, and the two measurement points are like a path. At this time, the measurement result read from the voltage comparator is a path. To solve this false path phenomenon caused by capacitance, the following two methods can be used: appropriately increase the measurement current to shorten the charging time, so that the charging process ends before reading the measurement results; Include a section to check true and false paths in the measurement software (see Section 5).

2. Influence of inductance

If there is inductance between the measured pins, it should be a circuit breaker relationship, but because the static resistance and impedance of the inductance is very small, the result measured by a multimeter is always a channel. Contrary to the situation of capacitance measurement, in the moment of analog switch closure, due to the inductance induced electromotive force, so the detection circuit acquisition speed can be used to judge the inductance correctly. But this is contradictory to the requirement of capacitance measurement.

3 Simulate the effect of switch jitter

In the actual measurement, it is found that the analog switch has a stable process from the open state to the closed state, which is manifested as the fluctuation of voltage VA, which makes the results of the initial several measurements inconsistent. Therefore, it is necessary to judge the results of the channel several times, and then confirm after the measurement results are consistent.

4. Confirmation and record of measurement results

Considering the above situations, in order to adapt to different measured objects, the software block diagram as shown in Figure 3 is used to confirm and record measurement results.

To eliminate the effect of capacitor element and analog switch jitter, the measurement time needs to be extended. To eliminate the effect of inductor element, the induced electromotive force needs to be determined in a very short time. For this purpose, two counters are set up in the program: the number of paths counter and the number of breaks counter. The number of paths N is set to eliminate the influence of false paths caused by capacitor charging at the moment when the analog switch is closed. That is, when the result of N paths is read accumulatively, the charging of the capacitor is generally finished, and the paths between the measured points are confirmed as paths. The number of circuit breakers n is set to eliminate the interference caused by analog switch jitter. Generally, it can be confirmed that the jitter of analog switch has ended when n times of circuit breakers are measured continuously. However, because the induced electromotive force of the inductance is the largest at the moment of the analog switch closing and then decreases rapidly, if the first and second measurement results are both open, it is confirmed as open. Due to the contradiction between several cases, the counter and delay value is determined according to the actual situation on the basis of weighing three cases.

Of course, when judging by the above procedure, if there is a small resistance, small inductance or large capacitance between the tested pins, it may be misjudged as a path. This problem is easy to be detected by the software in the measurement results. If the two ends of a two-pin element are in the same network, the above error of judgment may exist, and it will be excluded after confirmation.

conclusion

By analyzing the function and realization principle of PCB path detection circuit, this paper provides a new idea for the measurement of the path relationship between the pins on large-scale PCB components. The experiment shows that the detection circuit can measure and record the path relationship between the pins of each component on PCB efficiently, accurately and completely with the support of the measurement and navigation software (described in another article).